1. Field of the Invention
The present invention relates to zoom lens system which is used in video and digital cameras and the like having a high resolution.
2. Description of the Related Art
In recent years, along with demands for reducing the size of an image pick-up device and for increasing the density of pixels therein, a zoom lens system to be used in video and electronic still cameras and the like is also required to be smaller and to have an enhanced optical performance. In this regard, a conventional small-sized zoom lens system is known to have a two-lens-group arrangement which includes a negative first lens group and a positive second lens group, in this order from the object. However, if a zoom ratio exceeds 2, the power of each lens group becomes stronger, and thereby such a zoom lens system has difficulties in correcting various aberrations, and becomes more sensitive to even a minute manufacturing error of each lens element. Machining and assembling of lens elements therefore becomes more difficult.
As an attempt to solve the above problems, in the U.S. Pat. (hereinafter, U.S.P.) No. 5,543,969 and U.S. Pat. No. 5,912,771, a three-lens-group zoom lens system, which is constituted by a two-lens-group zoom lens system and a positive stationary lens group positioned on the object-side of the first lens group of the two-lens-group zoom lens system, is proposed, and thereby the three-lens-group zoom lens system attains a zoom ratio of about 3. The zoom lens system taught in U.S. Pat. No. 5,543,969 has a smaller F-number and a longer back focal distance; however, the overall length thereof is longer, and miniaturization thereof cannot be attained, since the number of the lens elements therein is large, and still further, at the short focal length extremity, a half angle-of-view is about 27xc2x0, which means that the focal length thereat is not short enough. Furthermore, the zoom lens system taught in U.S. Pat. No. 5,912,771 has a small number of lens elements, and has a large half angle-of-view of about 30xc2x0; however, a back focal distance thereof is short. If the zoom lens system of U.S. Pat. No. 5,912,771 were used in a single lens reflex camera, it would be difficult to secure a space for a prism to bifurcate an optical path to the view finder.
It is an object of the present invention to provide a zoom lens system, having the following features, to be used in small-sized video and digital cameras and the like:
a zoom ratio of about 2.5 to 4.0;
a half angle-of-view of about 35xc2x0 at the short focal length extremity;
an image-forming performance suitable for an image pick-up device having a high resolution;
a longer back focal distance; and
a shorter overall length.
In order to achieve the above mentioned object, there is provided a zoom lens system which includes a positive first lens group, a negative second lens group, and a positive third lens group, in this order from the object. Zooming is performed by moving the second lens group and the third lens group, and the zoom lens system satisfies the following conditions:
0.0 less than Fw/f1 less than 0.15xe2x80x83xe2x80x83(1)
0.75 less than |f2/f3| less than 1.0xe2x80x83xe2x80x83(2)
1.65 less than LD/Ft less than 2.25xe2x80x83xe2x80x83(3)
wherein
Fw designates the focal length of the entire lens system at the short focal length extremity;
Ft designates the focal length of the entire lens system a the long focal length extremity;
f1 designates the focal length of the first lens group;
f2 designates the focal length of the second lens group;
f3 designates the focal length of the third lens group; and
LD designates the distance, at the short focal length extremity, from the most object-side surface of the first lens group to the most image-side surface of the third lens group.
The third lens group is a three-sub-lens-group lens system of four lens-elements. The sub-lens groups include a positive lens element, a cemented sub-lens group having a positive lens element and a negative lens element, and a positive lens element, in this order from the object. The third lens group preferably satisfies the following conditions:
0.5 less than R1/R2 less than 2.0xe2x80x83xe2x80x83(4)
N3 less than 1.7xe2x80x83xe2x80x83(5)
v3 greater than 50xe2x80x83xe2x80x83(6)
wherein
R1 designates the radius of curvature of the object-side surface of the cemented sub-lens group in the third lens group;
R2 designates the radius of curvature of the image-side surface of the cemented sub-lens group in the third lens group;
N3 designates the averaged refractive index of the positive lens elements in the third lens group; and
v3 designates the averaged Abbe number of the positive lens elements in the third lens group.
For the purpose of simplifying the third lens group and miniaturizing the entire lens system, it is preferable to provide, in the third lens group, a positive lens element having at least one aspherical surface on which a positive power is weakened according to an increase of a distance from the optical axis. Due to the aspherical surface, in particular, spherical aberration and coma can effectively be corrected, and the third lens group can be simplified. Furthermore, the entire lens system can be miniaturized while a suitable optical performance is maintained.
The second lens group is a three-sub-lens-group lens system of three lens elements. The sub-lens groups include a negative meniscus lens element having a convex surface facing toward the object, a negative biconcave lens element, and a positive lens element having a sharp convex surface facing toward the object, in this order from the object. The second lens group preferably satisfies the following condition:
0.4 less than Rp/Rn less than 1.0xe2x80x83xe2x80x83(7)
wherein
Rp designates the radius of curvature of the object-side surface of the positive lens element in the second lens group; and
Rn designates the radius of curvature of the image-side surface of the negative biconcave lens element in the second lens group.
In the second lens group, the second lens element, i.e., the negative biconcave lens element, and the third lens element, i.e., the positive lens element having the sharp convex surface facing toward the object, can be cemented. When these two lens elements are cemented, a cemented sub-lens group preferably satisfies the following condition:
0.6 less than Fw/Rc less than 0.9xe2x80x83xe2x80x83(8)
wherein
Rc designates the radius of curvature of the cemented surface of the cemented sub-lens group in the second lens group.
Furthermore, in the case where the second lens group is arranged to be a focusing lens group which is made moveable along the optical axis to obtain an in-focus state for the object, the second lens group preferably satisfies the following condition:
|(m2wxe2x88x92m2t)/m2w| less than 0.2xe2x80x83xe2x80x83(9)
wherein
m2w designates a transverse magnification, at the short focal length extremity, of the second lens group when the zoom lens system is focused at infinity; and
m2t designates a transverse magnification, at the long focal length extremity, of the second lens group when the zoom lens system is focused at infinity.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-131303 (filed on May 12, 1999) which is expressly incorporated herein by reference in its entirety.